A new method to describe the three-body decaying states is developed. In this method, the Lippmann–Schwinger equation is combined with the complex scaling method to take the correct boundary condition into account. For the application, the E1 transition strength of 6 He is investigated. The energy and Dalitz distributions of the E1 transition are calculated, and the internal correlation of 6 He is discussed. As results, it is found that the 6 He → 5 He (3/2-)+n → 4 He +n+n sequential decay process is dominant in the E1 transition reaction of 6 He .